Fuel cells provide an environmentally friendly source of electrical power. One form of fuel cell used for generating electrical power, particularly for vehicle propulsion and for smaller scale stationary power generation, includes an anode channel for receiving a flow of hydrogen gas, a cathode channel for receiving a flow of oxygen gas, and a polymer electrolyte membrane (PEM) which separates the anode channel from the cathode channel. Oxygen gas entering the cathode reacts with hydrogen ions that cross the electrolyte to generate a flow of electrons. Environmentally safe water vapor is also produced as a byproduct.
Despite the great attractiveness of fueling future mobile and stationary power plants with hydrogen, severe technical and economic barriers are presented by the supply and storage of hydrogen (as either a compressed gas or a cryogenic liquid). A substantial amount of mechanical energy must be invested to compress and/or refrigerate hydrogen fuel being stored in any of these hydrogen storage systems. In the absence of any system for recovering energy as hydrogen fuel is released from storage, this energy loss seriously impairs the overall efficiency and economic viability of the so-called “hydrogen economy”, as popularly represented by distributed fuel cell energy systems using hydrogen generated by renewable energy sources such as solar photovoltaic power. In particular, the economic viability of hydrogen and natural gas as fuels particularly for vehicular propulsion has been gravely compromised by the loss of energy required to compress or liquefy these fuels in the fuel supply infrastructure. The prospects for wide application of hydrogen energy systems based on fuel cells could be greatly enhanced by development of a system for recovering “hydrogen storage energy” to improve overall energy efficiency.
Specifically, hydrogen may be stored at substantially ambient temperature as a compressed gas in high-pressure vessels, or in solid solution within a metal hydride canister. Hydrogen may alternatively be stored at low temperatures (e.g., about 77 K to about 200 K) as a compressed gas in contact with an adsorbent (e.g. active carbon), or at much lower temperature (˜20 K) as cryogenic liquid. Some researchers are currently investigating hydrogen storage at substantially ambient temperature as a compressed gas in contact with an advanced adsorbent (e.g. nanofiber or nanotube carbon).
Each of the above physical techniques for hydrogen storage requires a substantial investment of “hydrogen storage energy” (typically as compression energy) to achieve the required working storage pressure and to provide any required cryogenic refrigeration. There has been a lack of practical devices and methods for recovering hydrogen storage energy to enhance the performance and efficiency of the fuel cell power plant, particularly for small-scale fuel cell power plants. There are some examples of hydrogen storage energy being recovered for ancillary uses. One example involves recovering a portion of hydrogen storage energy for air conditioning for passenger compartment comfort, where hydrogen fuel being released from cryogenic storage may be used as a refrigerant. Another example involves using the endothermic heat of hydrogen release from a metal hydride as a heat sink.
One way to improve the performance of a PEM fuel cell system is oxygen enrichment of the air supplied to the cathode. Boosting the oxygen partial pressure over the fuel cell cathode will enhance fuel cell stack voltage efficiency at a given current density. Alternatively, oxygen enrichment can enable fuel cell operation at higher current density with reduced voltage drop, thus reducing the size and capital cost of the equipment.
Pressure swing adsorption (PSA) systems can provide a continuous supply of enriched oxygen while also removing any contaminant gas or vapor components that may be detrimental to the fuel cell. PSA systems (including vacuum pressure swing adsorption systems (VPSA)) separate gas fractions from a gas mixture by coordinating pressure cycling and flow reversals over an adsorber or adsorbent bed that preferentially adsorbs a more readily adsorbed gas component relative to a less readily adsorbed gas component of the mixture. The total pressure of the gas mixture in the adsorber is elevated while the gas mixture is flowing through the adsorber from a first end to a second end thereof, and is reduced while the gas mixture is flowing through the adsorbent from the second end back to the first end. As the PSA cycle is repeated the less readily adsorbed component is concentrated adjacent to the second end of the adsorber, while the more readily adsorbed component is concentrated adjacent to the first end of the adsorber. As a result, a “light” product (a gas fraction depleted in the more readily adsorbed component and enriched in the less readily adsorbed fraction, here oxygen and argon) is delivered from the second end of the adsorber, and a “heavy” product (a gas fraction enriched in the strongly adsorbed components, here nitrogen, water vapor, carbon dioxide, and any contaminants) is exhausted from the first end of the adsorber.
The conventional system for implementing pressure swing adsorption or vacuum pressure swing adsorption uses two or more stationary adsorbers in parallel, with directional valving at each end of each adsorber to connect the adsorbers in alternating sequence to pressure sources and sinks. This system is often cumbersome and expensive to implement due to the large size of the adsorbers and the complexity of the valving required. Further, the conventional PSA system makes inefficient use of applied energy because of irreversible gas expansion steps as adsorbers are cyclically pressurized and depressurized within the PSA process. Conventional PSA systems could not be applied to fuel cell power plants for vehicles, as such PSA systems are far too bulky and heavy because of their low cycle frequency and consequently large adsorbent inventory.
A serious challenge for oxygen enrichment by PSA or VPSA using nitrogen-selective zeolite adsorbents arises from the strongly hydrophilic nature of those adsorbents. Water adsorption from atmospheric humidity will deactivate the adsorbent. For continuously operating industrial PSA plants, this problem is solved by using the feed end of the adsorbent bed (typically loaded with alumina desiccant) to dry the feed air. In intermittent operation, adsorbed water in the desiccant layer may diffuse into the nitrogen-selective adsorbent zone and cause deactivation of that adsorbent during shutdown intervals. Hence, it is very desirable that as much water as possible be removed from the feed air before that air enters the PSA unit, in order to reduce the humidity challenge to satisfactory sustained operation of the PSA under intermittent operating conditions.